System and method for signal read-out using source follower feedback

1. A method for signal read-out comprising:

receiving, by an amplifier circuit, a positive differential input signal at a first subcircuit of the amplifier circuit, the first subcircuit comprising a source follower transistor;

receiving, by the amplifier circuit, a negative differential input signal at a second subcircuit of the amplifier circuit, the second subcircuit comprising a source follower transistor;

transmitting one of a pair of output signals as a first feedback signal to a capacitive feedback circuit of the first subcircuit, wherein the pair of output signals comprise an output signal of the first subcircuit and an output signal of the second subcircuit;

transmitting one of the pair of output signals as a second feedback signal to a capacitive feedback circuit of the second subcircuit, wherein

the first feedback signal is distinct from the second feedback signal;

determining a target voltage transfer ratio of the amplifier circuit in accordance with a sensitivity of an external input device and in accordance with a target output sensitivity of a signal read-out for the external input device;

adjusting a capacitance of the capacitive feedback circuit of the first subcircuit in accordance with the target voltage transfer ratio; and

adjusting a capacitance of a capacitive feedback circuit of the second subcircuit in accordance with the target voltage transfer ratio.

2. The method of claim 1 , further comprising:

selecting, in accordance with the target voltage transfer ratio, the first feedback signal from the pair of output signals; and

selecting, in accordance with the target voltage transfer ratio, the second feedback signal from the pair of output signals.

3. The method of claim 2 , wherein:

the transmitting the first feedback signal comprises:

determining a non-feedback voltage transfer ratio;

comparing the target voltage transfer ratio to the non-feedback voltage transfer ratio; and

selecting, when the target voltage transfer ratio is greater than the non-feedback voltage transfer ratio, the output signal of the first subcircuit as the first feedback signal.

4. The method of claim 1 , wherein each of the first subcircuit and the second subcircuit further comprises a super source follower (SSF) stage comprising the source follower transistor.

5. The method of claim 1 , wherein each of the first subcircuit and the second subcircuit further comprises:

a current bias transistor comprising a gate coupled to an output of the capacitive feedback circuit; and

a feedback node coupled to an input of the capacitive feedback circuit.

6. The method of claim 5 , wherein a configurable voltage transfer ratio of each of the first subcircuit and the second subcircuit varies in accordance with an amplitude of a current variation through the current bias transistor.

7. The method of claim 1 , wherein the positive differential input signal and the negative differential input signal are generated as differential outputs of an external micro-electro-mechanical system (MEMS) device.

8. The method of claim 7 , wherein:

the external MEMS device comprises an external microphone, the external microphone comprising a membrane capacitively coupled to a pair of back-plates; and

the differential outputs of the external MEMS device are generated at the pair of back-plates.

9. An amplifier circuit comprising:

a pair of subcircuits comprising a first subcircuit and a second subcircuit, each comprising:

a buffer amplifier; and

a feedback circuit comprising a feedback capacitor, wherein

the first subcircuit and the second subcircuit each generate a different output signal of a pair of output signals, the pair of output signals comprises a first output signal and a second output signal, and the amplifier circuit is configured for

receiving a positive differential input signal at the first subcircuit,

receiving a negative differential input signal at the second subcircuit,

transmitting the first output signal to the feedback circuit of the first subcircuit,

transmitting the second output signal to the feedback circuit of the second subcircuit,

adjusting, in accordance with a target voltage transfer ratio, a capacitance of the feedback circuit of each of the first subcircuit and the second subcircuit,

selecting, in accordance with the target voltage transfer ratio, the first output signal from a pair of output signals comprising an output of the first subcircuit and an output of the second subcircuit, and

selecting, in accordance with the target voltage transfer ratio, the second output signal from the pair of output signals, wherein the second output signal is distinct from the first output signal.

10. The amplifier circuit of claim 9 , wherein the positive differential input signal and the negative differential input signal are generated as differential outputs of an external micro-electro-mechanical system (MEMS) device.

11. The amplifier circuit of claim 10 , wherein:

the external MEMS device comprises an external microphone, the external microphone comprising a membrane capacitively coupled to a pair of back-plates; and

the differential outputs of the external MEMS device are generated at the pair of back-plates.

12. The amplifier circuit of claim 9 , wherein each of the first subcircuit and the second subcircuit further comprises a super source follower (SSF) stage comprising the buffer amplifier.

13. The amplifier circuit of claim 9 , wherein:

each of the pair of subcircuits further comprises:

a current bias circuit coupled in series with the buffer amplifier, the current bias circuit comprising a current bias transistor comprising a gate coupled to an output of the feedback circuit;

a subcircuit input node coupled to a gate of the buffer amplifier;

a subcircuit output node; and

a feedback node coupled to an input of the feedback circuit, wherein

a positive input terminal of the amplifier circuit is coupled to the subcircuit input node of the first subcircuit;

a negative input terminal of the amplifier circuit is coupled to the subcircuit input node of the second subcircuit;

a first output terminal is coupled to the subcircuit output node of the first subcircuit; and

a second output terminal is coupled to the subcircuit output node of the second subcircuit.

14. The amplifier circuit of claim 13 , wherein the current bias transistor of each of the pair of subcircuits comprises a p-channel transistor.

15. The amplifier circuit of claim 13 , wherein the current bias transistor of each of the pair of subcircuits comprises an n-channel transistor.

16. An amplifier circuit comprising:

a pair of subcircuits comprising a first subcircuit and a second subcircuit, each comprising:

a buffer amplifier; and

a feedback circuit comprising a feedback capacitor, the first subcircuit and the second subcircuit each generate a different output signal of a pair of output signals, the pair of output signals comprises a first output signal and a second output signal, and the amplifier circuit is configured for

receiving a positive differential input signal at the first subcircuit,

receiving a negative differential input signal at the second subcircuit,

transmitting the first output signal to the feedback circuit of the first subcircuit, and

transmitting the second output signal to the feedback circuit of the second subcircuit;

a current bias circuit coupled in series with the buffer amplifier, the current bias circuit comprising a current bias transistor comprising a gate coupled to an output of the feedback circuit;

a subcircuit input node coupled to a gate of the buffer amplifier;

a subcircuit output node; and

a feedback node coupled to an input of the feedback circuit, wherein

a positive input terminal of the amplifier circuit is coupled to the subcircuit input node of the first subcircuit,

a negative input terminal of the amplifier circuit is coupled to the subcircuit input node of the second subcircuit,

a first output terminal is coupled to the subcircuit output node of the first subcircuit,

a second output terminal is coupled to the subcircuit output node of the second subcircuit, and

a configurable voltage transfer ratio of each of the pair of subcircuits varies in accordance with an amplitude of a current variation through the current bias transistor.

17. The amplifier circuit of claim 16 , wherein the current bias circuit further comprises:

a current source transistor coupled in series with the buffer amplifier, the current source transistor comprising a gate coupled to an input of the current bias circuit; and

a plurality of cascode transistors coupled to the gate of the current source transistor and coupled in series with the current bias transistor.

18. The amplifier circuit of claim 17 , further comprising:

a voltage bias circuit configured to provide a plurality of bias voltages comprising a first bias voltage, a second bias voltage, and a third bias voltage, wherein:

the first bias voltage is coupled to the positive input terminal and to the negative input terminal; and

the plurality of cascode transistors comprises:

a p-channel cascode transistor coupled in series with the gate of the current source transistor, the p-channel cascode transistor comprising a gate coupled to the second bias voltage; and

a n-channel cascode transistor coupled in series with the p-channel cascode transistor and with the gate of the current source transistor, the n-channel cascode transistor comprising a gate coupled to the third bias voltage.

19. The amplifier circuit of claim 18 , wherein the feedback circuit of each of the pair of subcircuits comprises:

a plurality of feedback capacitors; and

a plurality of switches coupled to the feedback node and to the plurality of feedback capacitors.

20. An amplifier system, comprising:

a pair of subcircuits, each comprising:

a buffer amplifier;

a feedback circuit, comprising at least one feedback capacitor; and

a current bias circuit coupled in series with the buffer amplifier, the current bias circuit comprising

a current bias transistor comprising a gate coupled to an output of the feedback circuit,

a current source transistor coupled in series with the buffer amplifier, the current source transistor comprising a gate coupled to an input of the current bias circuit, and

a plurality of cascode transistors coupled to the gate of the current source transistor and coupled in series with the current bias transistor;

an input node coupled to a gate of the buffer amplifier;

an output node;

a feedback node coupled to an input of the feedback circuit;

a positive differential input terminal coupled to the input node of a first subcircuit of the pair of subcircuits;

a negative differential input terminal coupled to the input node of a second subcircuit of the pair of subcircuits;

a pair of output terminals comprising a first output terminal and a second output terminal, wherein

the feedback node of each of the pair of subcircuits is coupled to a different one of the pair of output terminals,

the first output terminal is coupled to the output node of the first subcircuit, and

the second output terminal is coupled to the output node of the second subcircuit; and

a voltage bias circuit configured to provide a plurality of bias voltages comprising a first bias voltage, a second bias voltage, and a third bias voltage, wherein

the first bias voltage is coupled to the positive differential input terminal and to the negative differential input terminal, and

the plurality of cascode transistors comprises

a p-channel cascode transistor coupled in series with the gate of the current source transistor, the p-channel cascode transistor comprising a gate coupled to the second bias voltage, and

a n-channel cascode transistor coupled in series with the p-channel cascode transistor and with the gate of the current source transistor, the n-channel cascode transistor comprising a gate coupled to the third bias voltage.

21. The amplifier system of claim 20 , wherein the positive differential input terminal and the negative differential input terminal are coupled to differential output terminals of an external micro-electro-mechanical system (MEMS) device.

22. The amplifier system of claim 21 , wherein:

the external MEMS device comprises an external microphone, the external microphone comprising a membrane capacitively coupled to a pair of back-plates; and

the differential output terminals of the external MEMS device are coupled to the pair of back-plates.

23. The amplifier system of claim 20 , wherein:

a capacitance of the feedback circuit of each of the pair of subcircuits is adjustable; and

the at least one feedback capacitor of each of the pair of subcircuits comprises a plurality of feedback capacitors.

24. The amplifier system of claim 23 , wherein the feedback circuit of each of the pair of subcircuits further comprises a plurality of switches coupled to the feedback node and to the plurality of feedback capacitors.

25. The amplifier system of claim 24 , wherein a configurable voltage transfer ratio of each of the pair of subcircuits varies in accordance with an amplitude of a current variation through the current bias transistor.

26. The amplifier system of claim 25 , wherein the current bias transistor of each of the pair of subcircuits comprises a p-channel transistor.

27. The amplifier system of claim 25 , wherein the current bias transistor of each of the pair of subcircuits comprises an n-channel transistor.